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Photosynthetic capacity, determined by light harvesting and carboxylation reactions, is a key plant trait that determines the rate of photosynthesis; however, in Earth System Models (ESMs) at a reference temperature, it is either a fixed value for a given plant functional type or derived from a linear function of leaf nitrogen content. In this study, we conducted a comprehensive analysis that considered correlations of environmental factors with photosynthetic capacity as determined by maximum carboxylation (Vc,m) rate scaled to 25°C (i.e., Vc,25; μmol CO2·m-2·s-1) and maximum electron transport rate (Jmax) scaled to 25°C (i.e., J25; μmol electron·m-2·s-1) at the global scale. Our results showed that the percentage of variation in observed Vc,25 and J25 explained jointly by the environmental factors (i.e., day length, radiation, temperature, and humidity) were 2-2.5 times and 6-9 times of that explained by area-based leaf nitrogen content, respectively. Environmental factors influenced photosynthetic capacity mainly through photosynthetic nitrogen use efficiency, rather than through leaf nitrogen content. The combination of leaf nitrogen content and environmental factors was able to explain ~56% and ~66% of the variation in Vc,25 and J25 at the global scale, respectively. Our analyses suggest that model projections of plant photosynthetic capacity and hence land-atmosphere exchange under changing climatic conditions could be substantially improved if environmental factors are incorporated into algorithms used to parameterize photosynthetic capacity in ESMs. [ABSTRACT FROM AUTHOR]

Abstract: Brazilian Eucalyptus plantations are some of the most productive forest plantations in the world, sustaining mean growth rates of 25Mgha−1 year−1 (50m3 ha−1 year−1) over the 4.7 million hectares planted across the country. To better understand forest productivity, studies at the stand scale need to be coupled with tree level evaluations of the production ecology (the assessment of wood production as a function of crown light absorption and light use efficiency). The soil clay content (≈20% to ≈40%), topography and historical land use of the experimental site generated a natural gradient in productivity. We measured (from 6 to 7years after planting) stem wood dry biomass growth and estimated light absorption and light use efficiency at the tree level with a three-dimensional array model (MAESTRA) in 12 plots within a seed-origin Eucalyptus grandis plantation. We investigated the hypothesis that dominant trees (the 20% largest) are more productive than non-dominant trees (the 20% smallest) as a result of greater light absorption and light use efficiency; and that with increasing productivity across plots, dominant trees would show larger increases in light use and light use efficiency in comparison to non-dominant trees. The 20% smallest of the trees averaged 10.6kg of stem wood dry biomass (1.6kg of stem wood growth during the last year of the rotation), compared with 185kg per stem wood in the 20% largest of trees (34kg of stem wood growth over the same period). The smallest trees contained 7.2% of the leaf area as compared to the largest trees (3.0m2 versus 41.7m2), and they absorbed only 6.7% as much light (2.2 versus 32.8GJyear−1). The smallest trees grew at about 4.7% of the rate of the largest trees, which is a smaller percentage than the difference in absorbed photosynthetically active radiation; therefore the light use efficiency was lower for the smallest trees (0.75kgGJ−1 versus 1.03kgGJ−1). Our results show the significant contribution of dominant trees to stand productivity and the importance of evaluating production ecology at the individual tree scale. [Copyright &y& Elsevier]

Abstract: The growth of wood in trees and forests depends on the acquisition of resources (light, water, and nutrients), the efficiency of using resources for photosynthesis, and subsequent partitioning to woody tissues. Patterns of efficiency over time for individual trees, or between trees at one time, result from changes in rates photosynthesis and shifts in the relative partitioning to wood. We measured the production ecology (stem growth, light interception, and light use efficiency) to explain patterns of growth among trees within plots through stand development, and tested three hypotheses: (1) dominant trees have higher light use efficiency than subordinate trees; (2) lower variation in the size distribution of trees within plots allows higher light use efficiency; and (3) uniform stand structure and high light use efficiency reduce the age-related decline in tree growth. The experiment used clonal plantations of Eucalyptus at four locations in eastern Brazil. Irrigation and fertilization treatments ensured the major resource limitation for tree growth would be light supply. The influence of variation in the sizes of trees within plots was tested by comparing plots with all trees planted in a single day (uniform treatment) with plots where planting was spread over 80 days (heterogeneous treatment). Light interception per tree was simulated with the MAESTRA model. Across sites, treatments and whole-rotation stand development, dominant trees showed higher rates of stem growth, light interception, and light use efficiency than subordinate trees (supporting the first hypothesis). For example, dominant trees (80th percentile rank) at the end of the rotation grew four-times faster than suppressed trees (20th percentile rank), as a result of 2.1-fold greater light interception, and 1.8-fold greater stem growth per unit of light interception. In some cases, greater variation among tree sizes within plots led to lower efficiency of light use by average-size trees, providing mixed evidence for the second hypothesis. Greater uniformity of sizes of trees within plots did not substantially mitigate the decline in stem growth from mid-rotation to the end of the rotation, refuting the third hypothesis. The high efficiency of dominant trees underscores the marginal contribution of subordinate trees to total stand growth, and should spur further work on thinning to increase growth and lengthen rotations for dominant trees. [Copyright &y& Elsevier]

The article presents an analysis concerning the adverse effects of drought's severity to the health condition of the forests in the southeast region of the U.S. It mentions that this catastrophe has caused significant changes in the climate which affects the life of the trees in the forests. In addition, this disaster has decreased the growth potentials of the trees and hastens mortality. The states affected include Georgia, Alabama, and Virginia and some species of trees are being examined their resiliency to the drought including oaks, mesophytes, and pines.

Abstract: American chestnut (Castanea dentata) is currently regarded as functionally extinct because of chestnut blight. To reintroduce blight-resistant American chestnut back to its historic range, it is imperative to understand the silvics and silviculture of the species. In an outdoor rainout shelter, we grew American chestnut seedlings at four levels of irradiance (4, 12, 32 and 100% of full sunlight) to examine how light intensity affects photosynthesis, growth, and biomass allocation. Net photosynthetic rate increased linearly with increasing irradiance while instantaneous water use efficiency peaked at 32% full sunlight, when seedlings were measured at their acclimated irradiance level. Height and diameter increased with increasing irradiance. However, seedlings only grew laterally under 4% full sunlight. Total biomass increased linearly with increasing irradiance and root to shoot ratio was lowest under 4 and 12% full sunlight. Regardless of irradiance level, >70% of total biomass was allocated to shoot growth. With increasing shade, specific leaf area significantly increased. These observed physiological and morphological light acclimation characteristics indicate that American chestnut is shade tolerant, which partially explains why the species has persisted in the understory for almost a century. The shade-tolerance and fast growing characteristics suggest that an underplanting-and-release or gap-phase regeneration approach would be a suitable silvicultural alternative to a clearcut-and-planting approach for the reintroduction of blight-resistant American chestnut. [Copyright &y& Elsevier]

Abstract: Effects of prescribed fires on the 1-year establishment of white oak seedlings were investigated on the Clemson Experimental Forest, South Carolina, USA. Three stands, each consisting of a burn and a control treatment of about 1ha in size, were examined in the study. On each burn and control treatment, six to eight dominant white oak trees were randomly selected along the slope and four 2-m radius quadrats were set up around each selected tree. Prescribed fire increased seedling biomass but it did not affect seedling mortality and root to shoot ratio. Effects of prescribed fire on seedling density, forest floor depth, and understory light intensity depended on stands, where burning increased seedling density in stands 1 and 3 but not in stand 2. Burning also reduced forest floor depth and increased understory light intensity in stands 1 and 3 but not in stand 2. Regression analyses indicated that forest floor depth and understory light intensity were significantly related to seedling density and biomass, with thinner forest floor and higher light intensity favoring the establishment of new oak seedlings. We conclude that burning can benefit the establishment of new white oak seedlings as long as it significantly reduces forest floor depth and increases understory light intensity. [Copyright &y& Elsevier]